1. Field of the Invention
The present invention generally relates to a long-distance transmission system and a device applicable thereto, and more particularly to an LT-NT long-distance transmission system in which a two-wire metallic subscriber line is used to mutually connect a network terminal (NT) which accommodates terminal equipment (TE)and a line terminal (LT).
The basic services of the ISDN (Integrated Services Digital Network) in Japan are designed and developed so that a two-wire metallic subscriber line used in the conventional analog telephone transmission is applied without any modification and high-transmission-rate data transmission is realized using such a subscriber line. At the initial stage of the development, it was planned to accommodate 99% of the total number of subscribers through digital transmission lines. After that, however, it was found that some areas accommodated in the analog system could not be accommodated in the digital system for various reasons. For example, there was a limitation resulting from loss of the main signal caused during the propagation of the main signal through the subscriber line. Further, there was a restriction concerning a feed of electricity to the NT (including TE). In addition, there has recently been an unexpected increase in the number of subscribers located at a long distance from the office facility (LT). Under the above situations, it is required to efficiently provide subscribers located in areas as described above with services.
2. Description of the Related Art
FIGS. 1, 2 and 3 show prior art. More particularly, FIG. 1 shows a network structure which conforms with the TTC standard (JT-G961) defined by the Telecommunication Technology Committee in Japan and which is provided by NTT (Nippon Telegraph and Telephone Corporation). The network shown in FIG. 1 includes a central office 100, a switch 6, a line terminal (LT) 50 accommodating network equipment (NT), a metallic subscriber line 1, a user""s house 200a, a network terminal (NT1) 30, ISDN standard terminal equipment (TE) 10, an existing analog telephone set (TEL) 5, a terminal adapter (TA) 4, an office building 200b, a network terminal (NT2) 60, a user""s house 200c located a long distance away from the central office 100, a central office terminal (CT) 61, a remote terminal (RT) 62, and an optical fiber cable 2. The network terminal (NT1) 30 has the functions of terminating the subscriber line 1, establishing a phase synchronization (frame synchronization and bit synchronization), changing the data transmission rate, and testing and protecting the system from an excessive voltage. The network terminal 30 can also be called a digital service unit (DSU). The terminal adapter (TA) 4 is equipped with functions of converting the protocol and changing the data transmission rate in order to connect the telephone set 5 to the network terminal 30. The network terminal (NT2) 60 has a PBX (Private Branch Exchange) function including a line connection control, switching and selection and a protocol processing function in order to accommodate the terminal equipment 10 and implement switching therebetween.
The central office 100 and the user""s house 200a are connected by a two-wire metallic subscriber line 1a as in the case of the conventional analog system. The possible transmission distance in the existing facility specification is 7 km without any repeater. The network terminal 30 and a plurality of terminal equipment 10 are connected by a four-wire metallic cable 3 (which is 100-200 m long) in a bus system. Symbols R, S, T and U denote reference points. The connection between the central office 100 and the user""s home 200c located a long distance away will be described later.
FIG. 2 schematically shows the line terminal 50, the network terminal 30 and the terminal equipment 10 and connections therebetween.
The line terminal 50 is configured as follows. A signal processing part 51 establishes a phase synchronization of the main signal and changes the transmission rate. A signal transmission circuit 52 sends a signal to the subscriber line 1. A signal reception circuit 53 receives a signal from the subscriber line 1, and includes the function of equalizing the main signal. A symbol T2 denotes a main signal transformer, and C2 is a capacitor for DC isolation. The capacitor C2 is not limited to the position shown in FIG. 2. An office feed part 54 remote-feeds electricity to the network terminal 30. A normal/reverse switch 55 switches the polarity in feeding. A power separation filter PSF2 is formed of a coil or the like, and realizes AC isolation.
The network terminal 30 is configured as follows. A symbol T1 denotes a main signal transformer, and C1 denotes a capacitance for DC isolation. The capacitor C1 is not limited to the position shown in FIG. 2. A board 31 has a main signal circuit part. A signal processing part 32 has the functions of establishing the phase synchronization of the main signal and changing the data transmission rate. A signal transmission circuit 33 sends a signal to the subscriber line 1. A variable equalizer 36 amplifies a signal received from the subscriber line 1 and compensates for (or equalizes) a deterioration of the received signal due to the characteristics of the transmission line (the amplitude characteristic, the phase characteristic and so on) which depend on the distance thereof. A symbol R denotes a reception circuit to the bus line 3, and a symbol T denotes a transmission circuit from the bus line 3. A symbol PSF1 denotes a power separation filter for AC isolation. A standby circuit 44 feeds DC electricity to the terminal equipment 10 when a call issued by the terminal equipment 10 is detected or the normal feeding of electricity to the network terminal 30 is performed. A call detection part 45 detects a call from the terminal equipment 10. A DC power source (SRG) 46 is a series regulation generator. A symbol DN denotes a diode which is connected in normal-connection fashion and implements the normal feed to the DC power source 46. A DC/DC converter 47 supplies DC electricity to the board (main signal circuit part) 31 and the terminal equipment 10 at the time of the reverse feed to the NT 30. A symbol DR denotes a diode which is connected in the reverse connection fashion and implements the reverse feed to the DC/DC converter 47. A terminal feed circuit 48 carries out the remote feed from the network terminal 30 to the terminal equipment 10.
The terminal equipment 10 is configured as follows. A symbol T denotes a transmission circuit which sends a signal to the bus line 3, and a symbol R denotes a reception circuit which receives a signal from the bus line 3. A signal processing part 11 processes the main signal. A DC/DC converter 12 receives the remote feed of electricity from the network terminal 30 and supplies the DC electricity to the terminal equipment 10.
AMI (Alternate Mark Inversion) code is applied to data communications between the terminal equipment 10 and the network terminal 30 and between the network terminal 30 and the line terminal 50. The AMI code is characterized in that the required bandwidth can be reduced and the DC component can be suppressed. The main signal level between the terminal equipment 10 and the network terminal 30 is equal to xc2x10.75 V and the main signal level between the network terminal 30 and the line terminal 50 is equal to xc2x10.6 V.
The channel bit rate supported by the ISDN basic services is such that 2B+D=144 kbps, and is formatted to the frame signal of a bit rate of 48 bits per frame (including various control bits) between the terminal equipment 10 and the network terminal 30. Hence, it takes 250 xcexcs to transmit one frame. Thus, the line bit rate between the terminal equipment 10 and the network terminal 30 is equal to 192 kbps, and is supported therebetween by two-way communications of the four-wire system.
The two-wire system is employed between the network terminal 30 and the line terminal 50. Thus, the channel bit rate of 144 kbps is supported by a time-division direction-control transmission system (Ping-Pong transmission system). Hence, the line bit rate between the network terminal 30 and the line terminal 50 is set equal to 320 kbps, which is more than twice the above-mentioned bit rate of 144 kbps, taking into account a transmission delay and a guard time defined between the consecutive frames. The time-division direction-control transmission system is also called a TCM (Time Compression Multiplexing) transmission system.
The network terminal 30 (and the terminal equipment 10) in the ISDN basic services is supplied with power necessary for communications from the central office side (line terminal 50). Hence, communications continue to take place in the event of an emergency such as breakdown of a commercial power supply.
A brief description will now be given of the remote feed control of the line terminal 50. The normal feed (L1=plus, L2=minus) from the line terminal 50 is carried out at the time of the standby of the network terminal 30 (when no communications take place). At that time, in the network terminal 30, the normal diode DN is turned on and the remote feed is applied to the series regulation source 46. Then, the series regulation source 46 outputs power as much as the remote feed to the standby circuit 44 and the terminal equipment 10 (over 40V and 420 mW).
When a call takes place from the terminal equipment 10 in the above-mentioned state, the call detection circuit 45 detects the above call, and notifies the line terminal 50 of the detection of a call by turning ON a loop circuit (not shown) or the like.
In the line terminal 50, the loop-ON is detected by the office feed part 54, which thus activates the normal/reverse switch 55. Hence, the feed is switched to the reverse feed (L1=minus, L2=plus) from the normal feed. At this time, the reverse diode DR of the network terminal 30 is turned ON and the DC/DC converter 47 is supplied with the remote feed. Then, the DC/DC converter 47 outputs power as much as the remote feed to the board (main signal processing circuit) 31 (5 V, 250 mW) and power as much as the remote feed to the terminal equipment 10 (over 40 V, 420 mW).
FIG. 3B shows a feed characteristic of the office feed part 54 of the line terminal 50.
Referring to FIG. 3B, the normal feed is defined so that a feed current IL is reduced in order to suppress power loss due to a line resistance RL of the metallic subscriber line 1 and a constant-voltage feed takes place. The constant voltage is equal to, for example, 60 Vxc2x15%. FIG. 3B shows that the minimum value of the output voltage Vo of the constant-voltage feed is 57 V. The input voltage Vi of the network terminal 30 is decreased as the distance (line resistance RL) from the line terminal 50 increases, as there is a little power loss due to the line resistance RL. However, the network terminal 30 is required to supply the terminal equipment 10 with power of 40 V (over 420 mW) even during the normal feed. Thus, a DC current as high as 11-15 mA is made to flow through the metallic subscriber line 1.
The reverse feed feeds electricity so that the network terminal 30 (and the terminal equipment 10) which is in communication is not affected by power loss due to the line resistance RL of the metallic subscriber line 1 and a constant-current feed is carried out. The constant current IL is equal to, for example, 35.1 mA. Hence, there is a maximum power loss due to the line resistance RL. When the line resistance is equal to 812 ohms which corresponds to the maximum value in the conventional standards, a power loss caused is equal to 1000 mW (=(35.1 mA)2xc3x97812 xcexa9).
The network terminal 30 is equipped with a constant-voltage receiving circuit (not shown) provided in the DC/DC converter 47 so that the input voltage Vi becomes constant (for example, 28.5 V) at the time of the reverse feed. Hence, the DC/DC converter 47 can always obtain an input power Pi of approximately 1000 mW (Pi=ViIL=28.5 Vxc3x9735.1 mA≈1000 mW). The input power Pi is separated into a conversion loss in the DC/DC converter 47 (which is approximately equal to 300 mW with a transformation efficiency of 70%), the self-operation source of the network terminal 30 (about +5 V, 250 mW), and a power fed to the lower terminals (such as terminal equipment 10) which conforms with the TTC standardized regulations (over 40 V, 420 mW).
At the initial stage of the development of the ISDN basic services, it was planned to accommodate 99% of the total number of subscribers through digital lines. After that, it was found that some areas accommodated in the analog system could not be accommodated in the digital system for various reasons. For example, there was a limitation resulting from loss of the main signal caused when the main signal was transmitted over the line. Further, there was a restriction concerning a feed of electricity to the NT (including TE). The above will be described in detail below.
FIG. 3A shows an AC loss characteristic of the metallic subscriber line 1. Generally, the loss of the metallic line 1 increases based on fxc2xd where f denotes frequency ({square root over (f)} characteristic). Further, the AC loss resulting from the distributed constant increases as the distance becomes longer. The AC loss is generally evaluated by loss in the given band of the main signal in which the signal power is concentrated.
Conventionally, the upper limit of the AC loss in the given band (around 160 kHz) is set up to 50 dB. Thus, the network terminal 30 (line terminal 50) cannot maintain a satisfactory signal quality in a long-distance accommodation in which the line loss exceeds the limitation of the main signal (50 dB). Hence, communications are no longer possible.
A description will be given, with reference to FIG. 3B, of the restriction due to the limitation on the remote feed.
The limitation on the remote feed depends on the reverse feed which causes a large line loss. At the time of the reverse feed, when it is assumed that the feed current It is equal to 35.1 mA and the voltage Vi necessary at the input of the network terminal 30 is equal to 28.5 V, the tolerable line resistance RL of the line 1 is obtained as follows by a back calculation from the maximum lower limit (57 V) of the feed output voltage Vo of the line terminal 50:                               R          L                =                                            (                              Vo                -                Vi                            )                        /            I                    ⁢                      xe2x80x83                    ⁢          t                                        =                                                            (                                  57                  -                  28.5                                )                            /              35.1                        xc3x97                          10                              -                3                                              ≈                      812            ⁢                          xe2x80x83                        ⁢                          Ω              .                                          
Hence, the network terminal 30 cannot receive expected power from the line terminal 50 and cannot supply the terminal equipment 10 with the regulated power in a long-distance accommodation over the limit value of the line resistance RL. In such cases, communications no longer take place.
As described above, the conventional art does not provide some areas with digital communication services due to the limitation on the main signal caused by the line loss and the restriction caused by the limitation of the power feed to the network terminal 30 (which includes the terminal equipment 10).
It may be possible to increase the feed voltage of the line terminal 50 in order to improve the limit value of the line resistance. However, such a measurement requires new facilities to be installed in the line terminal 50 and an increased cost of development and installation.
Turning to FIG. 1 again, the prior art employs the central office terminal (CT) 61 and a remote terminal (RT) 62, which terminals are provided between the central office 100 and the remote users 200c in order to enable the long-distance accommodation. The terminals 61 and 62 are coupled with each other through an optical subscriber line 2 (having a bit rate of, for example, 1.5/6.3 Mbps).
However, remote base stations such as the terminals 61 and 62 have an extremely high cost. In a case where a large number of subscribers offsetting the extremely high cost is not expected, the remote base stations are not liable to be installed in practice. Thus, in actuality, the digital communication services cannot be presented to all areas studded with subscribers from technical and economical viewpoints.
It is a general object of the present invention to eliminate the disadvantages of the prior art.
A more specific object of the present invention is to provide a long-distance transmission system and device capable of efficiently realizing long-distance digital transmission between the line terminal and the network terminal without any modification of main parts of the existing line terminal and the network terminal.
The above objects of the present invention are achieved by a long-distance transmission system comprising: a network terminal that accommodates terminal equipment; and a line terminal connected to the network terminal via a two-wire metallic subscriber line, the network terminal comprising an amplifier, which amplifies a transmission level of a transmission signal to be transmitted to the line terminal via the two-wire metallic subscriber line on the basis of a characteristic of the two-wire transmission line. Hence, the AC loss limit value can be improved without adding any substantial modification to the existing line terminal and long-distance communications can take place with high reliability.
The long-distance transmission system may be configured so that the amplifier amplifies the transmission level of the transmission signal so that the transmission signal is applied to the two-wire metallic subscriber line at a level exceeding an AC loss limit of the two-wire metallic subscriber line. Hence, the metallic subscriber line between the network terminal and the line terminal can be extended.
The long-distance transmission system may be configured so that the network terminal comprises a circuit which adjusts a waveform of the transmission signal so that an adjusted waveform thereof is suitable for the characteristic of the two-wire metallic subscriber line. The transmission signal deteriorates during the propagation through the metallic subscriber line, and the waveform thereof is deformed. The deterioration of the transmission signal depends on the frequency characteristic of the metallic line, which depends on, for example, the diameter of the metallic subscriber line. By adjusting the waveform of the transmission signal, it becomes possible to improve the equalizing performance and suppress distortion of the waveform of the transmission signal.
The long-distance transmission system may be configured so that the network terminal comprises a circuit which adjusts the transmission level of the transmission signal. The adjustment of the transmission level makes it possible to improve the equalizing performance and suppress distortion of the waveform of the transmission signal. This will be enhanced when both the transmission level and the waveform are adjusted.
The long-distance transmission system may be configured so that the network terminal comprises another amplifier which amplifies a reception level of a signal which is received from the line terminal via the two-wire metallic subscriber line. The use of the above-mentioned another amplifier contributes to improving the AC loss limit value.
The long-distance transmission system may be configured so that the network terminal and the terminal equipment are fed with electricity from a local source applied to the network terminal and thus a remote feed power supplied via the two-wire metallic subscriber line from the line terminal is terminated by a resistor having a reduced resistance value. Even if the network equipment is not fed with sufficient electricity from the line terminal via the metallic subscriber line due to an extension thereof (increase in loss of the line), the network equipment and the terminal equipment can be fed with sufficient power from the local source. Further, power which was originally to be supplied from the line terminal but is no longer needed due to the local source can be assigned to the loss of the metallic subscriber line. Hence, the metallic subscriber line can be extended definitely. In addition, the termination with a reduced resistance value contributes to stable operations of various functions of the network terminal such as transmission of the main signal, opening/closing a loop and changing the polarity of the line.
The long-distance transmission system may be configured so that the network terminal is fed with electricity from the (first) line terminal via the two-wire metallic subscriber line and the terminal equipment is fed with electricity from another line terminal via another (second) two-wire metallic subscriber line. The first line terminal is sufficient to cover electricity to be fed to the network terminal only, and the second line terminal is sufficient to cover electricity to be fed to the terminal equipment only. Conventionally, one line terminal is required to cover electricity to be fed to both the network terminal and the terminal equipment. Hence, each of the first and second line terminals has a reduced burden of supply of electricity. This means that power which becomes unnecessary to feed the network terminal and the terminal equipment is assigned to the loss of the metallic subscriber lines, which are allowed to have increased distances. In the above structure, the first and second line terminals are not required to be substantially modified.
The long-distance transmission system may be configured so that the circuit adjusts the waveform of the transmission signal so that a pulse width of the transmission signal is changed. The characteristic of the two-wire metallic subscriber line dependent on, for example, the diameter of the line, deforms the waveform of the transmission signal. This deformation can be corrected by adjusting the pulse width of the transmission signal.
The long-distance transmission system may be configured so that the network terminal and the terminal equipment are fed with electricity from the line terminal via the two-wire metallic subscriber line in either a normal feed or a reverse feed depending on whether the network terminal is in a working state or a standby state and is always fed, in the normal feed, with electricity from the other line terminal via another two-wire metallic subscriber line. The above another line terminal is sufficient to cover electricity to be supplied to only the terminal equipment, and is thus equipped with the normal feed.
The above-described objects of the present invention are also achieved by a network terminal connectable to a two-wire metallic subscriber line, comprising: a transmission circuit that outputs a transmission signal; and an amplifier part which amplifies a transmission level of the transmission signal which is to be sent to the two-wire metallic subscriber line. Hence, the AC loss limit value can be improved without adding any substantial modification to the existing line terminal and long-distance communications can take place with high reliability.
The network terminal may be configured so that the amplifier part amplifies the transmission level of the transmission signal so that the transmission signal is applied to the two-wire metallic subscriber line at a level exceeding an AC loss limit of the two-wire metallic subscriber line. Hence, the metallic subscriber line between the network terminal and the line terminal can be extended.
The network terminal may be configured so that it further comprises a circuit which adjusts a waveform of the transmission signal so that an adjusted waveform thereof is suitable for a characteristic of the two-wire metallic subscriber line. The transmission signal deteriorates during propagation through the metallic line, and the waveform thereof is deformed. The deterioration of the transmission signal depends on the frequency characteristic of the metallic line, which depends on, for example, the diameter of the metallic line. By adjusting the waveform of the transmission line, it becomes possible to improve the equalizing performance and suppress deterioration of the waveform of the transmission signal.
The network terminal may further comprise a circuit which adjusts the transmission level of the transmission signal. The adjustment of the transmission level makes it possible to improve the equalizing performance and suppress deterioration of the waveform of the transmission signal. This will be enhanced when both the transmission level and the waveform are adjusted.
The network terminal may further comprise a circuit which adjusts the transmission level of the transmission signal so that both the transmission level and the waveform can be adjusted. It is possible to prevent the transmission signal from deteriorating even when it is propagated through the metallic subscriber line having an extended distance.
The network terminal may further comprise a receive amplifier which amplifies a reception level of a signal which is received from a line terminal via the two-wire metallic subscriber line. The use of the above-mentioned receive amplifier contributes to improving the AC loss limit value.
The network terminal may further comprise a local feed part which feeds the network terminal and terminal equipment connected thereto with electricity from a local source applied to the network terminal, so that a remote feed power supplied via the two-wire metallic subscriber line from the line terminal is terminated by a resistor having a reduced resistance value. Even if the network equipment is not fed with sufficient electricity from the line terminal via the metallic subscriber line due to an extension thereof (increase in loss of the line), the network equipment and the terminal equipment can be fed with sufficient power from the local source. Further, power which was originally to be supplied from the line terminal but is no longer needed due to the local source can be assigned to the loss of the metallic subscriber line. Hence, the metallic subscriber line can be extended definitely. In addition, the termination with a reduced resistance value contributes to stable operations of various functions of the network terminal such as transmission of the main signal opening/closing a loop and changing the polarity of the line.
The network terminal may further comprise: a first feed part which receives electricity for the network terminal from a first line terminal via the two-wire metallic subscriber line; and a second feed part which receives electricity for terminal equipment connected to the network terminal from a second line terminal via a second two-wire metallic subscriber line. The first line terminal is sufficient to cover electricity to be fed to the network terminal only, and the second line terminal is sufficient to cover electricity to be fed to the terminal equipment only. Conventionally, one line terminal is required to cover electricity to be fed to both the network terminal and the terminal equipment. Hence, each of the first and second line terminals has a reduced burden of supply of electricity. This means that power which becomes unnecessary to feed the network terminal and the terminal equipment is assigned to the loss of the metallic subscriber lines, which are allowed to have increased distances. In the above structure, the first and second line terminals are not required to be substantially modified.
The network terminal may be configured so that the circuit adjusts the waveform of the transmission signal so that a pulse width of the transmission signal is changed. The characteristic of the two-wire metallic subscriber line dependent on, for example, the diameter of the line, deforms the waveform of the transmission signal. This deformation can be corrected by adjusting the pulse width of the transmission signal.
The network terminal may be configured so that: the amplifier part is a part of a transformer via which the network terminal is connected to the two-wire metallic subscriber line; and the part of the transformer has a winding ratio with which the transmission level of the transmission signal can be boosted. Hence, the transmission level can be increased by a simple structure. The amplifier part is not limited to the above but may be formed by a current amplifier or a voltage amplifier. Further, the amplifier part may be configured by adjusting given resistors used in the existing network terminal and connected to the power supply system.
The network terminal may be configured so that it further comprises: a first circuit which generates a plurality of waveforms of the transmission signal; and a second circuit which selects one of the plurality of waveforms suitable for a characteristic of the two-wire metallic subscriber line. The individual two-wire metallic subscriber lines have respective frequency v. AC loss characteristics. The above configuration makes it possible to select one of the waveforms of the transmission signal most suitable for the existing metallic subscriber line, so that the transmission signal can be propagated through the metallic subscriber line having an extended distance with a reduced deterioration.
The network terminal may further comprise a third circuit which adjusts the transmission level of the transmission signal so that an adjusted transmission level is suitable for the two-wire metallic subscriber line. Hence, the transmission signal can be propagated through the metallic subscriber line having an extended distance with a further reduced deterioration.
The network terminal may further comprise: a first circuit which generates a plurality of waveforms of the transmission signal; a second circuit which selects, in response to a control signal, one of the plurality of waveforms suitable for a characteristic of the two-wire metallic subscriber line; a third circuit which adjusts the transmission level in response to the control signal; and a fourth circuit which supplies the control signal to the second and third circuits. With a simple structure, it becomes possible to select the waveform of the transmission signal optimal to the metallic subscriber line and to thus prevent the waveform from deteriorating during propagation therethrough.
The network terminal may be configured so that: the fourth circuit includes a memory which stores items of data related to combinations of the waveforms and adjustable transmission levels; and one of the items of data suitable for the two-wire metallic subscriber line is read from the memory and applied to the second and third circuits as the control signal. With a simple structure, it becomes possible to select the waveform and transmission level of the transmission signal optimal to the metallic subscriber line and to thus prevent the waveform from deteriorating during propagation therethrough.
The network terminal may be configured so that: the items of data are sequentially selected one by one and are applied to the second and third circuits; and one of the items of data used when a receive system of the network terminal is pulled in synchronization based on data received via the two-wire metallic subscriber line is selected as the control signal. Even if the characteristic of the metallic subscriber line is unknown, it is possible to determine the waveform and transmission level of the transmission signal suitable for the metallic line.
The network terminal may be configured so that the above-mentioned one of the items of data selected as the control data is stored in the memory. Hence, communications can take place by using the control data stored in the memory so that highly reliable communications can be obtained.
The network terminal may further comprise a series regulator source which transforms the electricity from the second two-wire metallic subscriber line to power to be supplied, as a normal feed, to terminal equipment connected to the network terminal. The normal feed is a constant-voltage feed having comparatively small line loss, and the voltage drop at a power receiving terminal of the network terminal is small. Hence, a series regulator source which needs an input voltage greater than the output voltage (for example, 40 V) can be used. Generally, the series regulator source is a comparatively simple and less-expensive power source.
The network terminal may be configured so that it further comprises a DC/DC converter which converts the electricity from the second two-wire metallic subscriber line to power to be supplied, as a normal feed, to terminal equipment connected to the network terminal. Generally, the DC/DC converter is capable of deriving a desired output voltage (for example, 40 V) from any input voltage as long as a given power condition is satisfied. Hence, the use of the DC/DC converter enables both the normal feed and-the reverse feed (constant-current power feed).
The network terminal may be configured so that the receive amplifier comprises an amplifier having a flat gain characteristic. Generally, the AC loss of the metallic subscriber line increases nonlinearly as the length thereof increases. However, there are some cases where the AC loss of the metallic subscriber line substantially increases linearly as the distance increases. In these cases, an amplifier having a flat gain characteristic within a given frequency band at which power is concentrated can be used.
The receive gain of the main signal can also be accomplished by increasing the gain of the existing variable equalizer. However, it is not easy to modify the existing variable equalizer for the following reasons. First, the existing equalizer is adjusted so as to compensate for (equalize) a line loss up to 50 dB at maximum. Second, the existing equalizer is a part of an LSI device and is packaged with other circuits. In this regard, the above structure newly employs the receive amplifier.
Theoretically, the reception-side winding ratio of the transformer via which the metallic subscriber line is connected can be changed to increase the receive gain. In this case, if the transmission-side winding ratio of the above transformer is also adjusted so as to obtain an increased transmission gain, an excessive signal component is transferred from the transmission side of the transformer to the reception side. In this case, the LSI devices provided in the reception system of the network terminal may be damaged. For the above reason, the receive amplifier is preferably provided separated from the transformer while the transmit amplifier can be part of the transformer (adjustment of the transmission-side winding ratio).
The network terminal may be configured so that the receive amplifier comprises an amplifier having a slant gain characteristic. If the AC loss characteristic of the metallic subscriber line is changed in a complex fashion due to an extension thereof, it is preferable to use an amplifier having a slat gain characteristic, namely, an amplifier having an equalizing function which satisfies the Nyquist distortionless characteristic. Hence, it is possible to appropriately improve the limitation on the main signal due to the line loss.
The network terminal may further comprise a reset circuit which resets a given part of the network terminal in a standby mode by detecting a polarity of the two-wire metallic subscriber line in feeding electricity. If the given part of the network terminal (such as a main signal circuit) is in the standby mode (at the time of the normal feed) and is continuously supplied with electricity, the operation sequence of the given part may be affected. In contrast, the reset circuit resets the given part that is in the standby mode. Hence, the above problem does not occur.